Increasing interest in communication media, such as the Internet, electronic presentations, voice mail, and audio-conference communication systems, is increasing the demand for high-fidelity audio and communication technologies. Currently, individuals and businesses are using these communication media to increase efficiency and productivity, while decreasing cost and complexity. For example, audio-conference communication systems allow one or more individuals at a first location to simultaneously converse with one or more individuals at other locations through full-duplex communication lines in nearly real time, without wearing headsets or using handheld communication devices. Typically, audio-conference communication systems include a number of microphones and loudspeakers, at each location, that can be used by multiple individuals for sending and receiving audio signals to and from other locations.
In many audio-conference communication systems, audio signals carry a large amount of data, and employ a broad range of frequencies. Modern audio-conference communication systems attempt to provide clear transmission of audio signals, free from perceivable distortion, background noise, and other undesired audio artifacts. One common type of undesired audio artifact is an acoustic echo. Acoustic echoes can occur when a transmitted audio signal loops through an audio-conference communication system due to the coupling of microphones and speakers at a location. FIG. 1 shows a schematic diagram of an exemplary, two-location, audio-conference communication system. In FIG. 1, the audio-conference communication system 100 includes a near room 102 and a far room 104. Sounds, such as voices, produced in the near room 102 are detected by a microphone 106, and sounds produced in the far room 104 are detected by a microphone 108. The microphones 106 and 108 are transducers that convert the sounds into continuous analog signals that are represented by x(t) and y(t), respectively, where t is time.
The microphone 106 can detect many different sounds produced in the near room 102, including sounds output by the loudspeaker 114. An analog signal produced by the microphone 106 is represented by:y(t)=s(t)+f(x(t))+v(t)where
s(t) is an analog signal representing sounds produced in the near room 102,
v(t) is an analog signal representing noise, or extraneous signals created by disturbances in the microphone or communication channel 110, that, for example, may produces an annoying buzzing sound output from the loudspeaker 116, and
f(x(t)) is an analog signal that represents an acoustic echo.
The acoustic echo f(x(t)) is due to both acoustic propagation delay in the near room 102 and a round-trip transmission delay of the analog signal x(t) over the communication channels 110 and 112. Sounds generated by the analog signal y(t) are output from loudspeaker 116 in the far room 104. Depending on the amplification, or gain, in the amplitude of the signal y(t) and the magnitude of the acoustic echo f(x(t)), a person speaking into the microphone 108 in the far room 104 may also hear an annoying, high-pitched, howling sound emanating from loudspeaker 116 as a result of the sound generated by the acoustic echo f(x(t)).
Designers and manufacturers of audio-conference communication systems have attempted to compensate for acoustic echoes in various ways. One compensation technique employs a filtering system that reduces the acoustic echo. Typically, filtering systems employ adaptive filters that adapt to changing conditions at an audio-signal-receiving location. However, stand alone adaptive filters either fail to account for, or are often slow to adapt to, the widely varying acoustic path between loudspeaker and microphone. As a result, transmitted signals encoding conversations may be interrupted by numerous, brief periods of silence. Designers, manufacturers, and users of audio-conference communication systems have recognized a need for acoustic echo methods and systems that can reliably remove an acoustic echo from audio signals and that can rapidly adapt to the changing conditions at both audio-signal-receiving locations.